The Great 20th Century Drying of Africa

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The Great 20th Century Drying of Africa
Jim Hurrell, Marty Hoerling, Jon Eischeid Adam
Phillips, Taiyi Xu, Gary Bates Climate Analysis
Section, NCAR Climate Diagnostics Center, NOAA
Ninth Annual CCSM Workshop Climate Variability
Working Group 9 July 2004, Santa Fe
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Motivation

• Increasing evidence of the controlling effect of
  global SST variations
• on regional climate change over the 20th Century
• On the cause of the 1930s Dust Bowl
• (Schubert, Suarez, Pegion, Koster,
  Bacmeister Science 2004)
• The Perfect Ocean for Drought
• (Hoerling and Kumar Science 2003)
• Tropical Origins for Recent North Atlantic
  Climate Change
• (Hurrell, Hoerling, Phillips, Bates, Xu
  Climate Dynamics 2004)
• Oceanic Forcing of Sahel Rainfall
• (Giannini, Saravanan, Chang Science 2003)

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Atmospheric GCM Integrations Monthly evolving
global SST and sea ice (fixed solar
insolation, aerosols and trace-gas composition)
80 members NCAR (CAM2) (15)
ARPEGE (3)
(8)
NASA (NSIPP-1) (23)
ECHAM (3) (10)
ECHAM (4.5) (24)

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Coupled Integrations Unforced Control CCSM2
Last 650 of 1000 years
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Coupled Integrations Unforced Control CCSM2
Last 650 of 1000 years
Forced (Dutch Challenge Experiment) CCSM1.4
62 member ensemble
1940-2000 Prescribed GHG, sulfate aerosols,
solar radiation, volcanic
aerosols 2001-2080 Business as usual GHG
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Seasonal Cycle of Mean African Rainfall
First Harmonic
Observed
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Seasonal Cycle of Mean African Rainfall
First Harmonic
Observed
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Seasonal Cycle of 50-yr Seasonal Rainfall
Trends (1950-1999)
Observed
(3-month overlapping seasons)
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Seasonal Cycle of 50-yr Seasonal Rainfall
Trends (1950-1999)
Observed
FMA
JAS
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Time Series of Regional African
Rainfall (1950-1999)
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PDF of 50-yr Seasonal Rainfall Trends (1950-1999)
South Africa FMA
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PDF of 50-yr Seasonal Rainfall Trends (1950-1999)
South Africa FMA
Sahel JAS
Observed
Forced CCSM
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Preliminary Conclusions

• African drying since 1950 has followed the
  seasonal migration of
• monsoon rains
• The drying has been attributable to the temporal
  evolution of global SSTs
• Sub-Saharan drought has been strongly driven by
  the warming of the
• South Atlantic relative to the North Atlantic
  Ocean
• The progressive warming of the Indian Ocean has
  played a major role
• in the austral summer drought over southern
  Africa
• Experiments performed to date do not sample all
  SST influences known
• to be important for African rainfall
  variability
• Coupled climate models do not produce 50-year
  drying trends as
• large as observed